1. Field of the Invention
The present invention generally relates to conditioning polishing pads on chemical mechanical planarization systems (CMP, also referred to as chemical mechanical polishing systems). More particularly, the present invention relates to a closed-loop control system whereby an amount of pad conditioning is related to a measured roughness of the polishing pad.
2. Description of the Background Art
Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. A common technique for forming the circuitry on a semiconductor is photolithography. Part of the photolithography process requires that a special device (e.g., typically a photolithographic stepper or scanner) focus on the wafer to project an image of the circuit on the wafer. The ability of the device to focus on the surface of the wafer is often adversely affected by inconsistencies or unevenness on the surface of the wafer. This ability to focus becomes more critical as design rules become progressively smaller since the depth-of-focus becomes more limited as the resolution limit is decreased. Semiconductor wafers are also commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are formed through deposited dielectric layers to connect with a next level of the circuit. Each layer of the circuit can create or add unevenness to the wafer. Typically, a wafer is planarized before generating the next circuit layer.
CMP techniques are used to planarize a bare wafer and many or all layers added thereafter. Available CMP systems often use a rotating wafer holder that brings the wafer into contact with a polishing pad moving in the plane of the wafer surface to be planarized. A polishing fluid, such as a chemical polishing agent or slurry containing microabrasives, is applied to the polishing pad to polish the wafer. The wafer holder rotates while the wafer is pressed against the rotating polishing pad to planarize the wafer.
During the planarization process, the properties of the polishing pad will change. Slurry particles and polishing byproducts accumulate on the surface of the pad. Polishing byproducts and morphology changes on the pad surface affect the properties of the polishing pad and cause the polishing pad to suffer from a reduction in both its polishing rate and performance uniformity. To maintain a consistent pad surface, provide microchannels for slurry transport, and remove debris or byproducts generated during the CMP process, polishing pads are typically conditioned. Pad conditioning restores the properties of the polishing pad by re-abrading or otherwise restoring the surface of the polishing pad. This conditioning process enables the pad to maintain a stable removal rate while polishing a substrate or planarizing a deposited layer and lessens the impact of pad degradation on the quality of the planarized wafer.
One method used for conditioning a polishing pad uses a rotary disk embedded with diamond particles to roughen the surface of the polishing pad. Typically, the disk is brought against the polishing pad and rotated about an axis perpendicular to the plane of the polishing pad while the polishing pad is rotated. The diamond-coated disks produce microgrooves on the surface of the polishing pad.
Currently, pad conditioners are simply applied to a polishing pad for a given time at a given force and rotational speed. However, the life of a polishing pad is a key factor in the cost of a CMP process. Overconditioning the polishing pad erodes the surface and reduces the life of the polishing pad as well as increases processing time. Conversely, underconditioning the polishing pad produces incomplete planarization often accompanied by reduced yields. Accordingly, advances in methods and apparatuses for conditioning polishing pads used in the chemical mechanical planarization of semiconductor wafers are necessary to improve polishing pad life, increase wafer throughput, and reduce costs.